The present invention relates to a servo motor apparatus for a printer, facsimile transceiver or the like. In such an apparatus a theoretically optimum rotational speed of a motor shaft is calculated as a function of the angular distance of the motor shaft from the present step or position to the new desired command step or position. A speed sensor connected to the motor shaft produces pulses corresponding in period or width to the actual motor shaft speed. The theoretical and actual speeds are compared to produce a speed difference signal which is in turn used to calculate a drive pulse time period or pulse width for energizing the motor for movement from the present step to the next step. The theoretical speed is progressively reduced as the motor shaft approaches the command position. The motor may be energized in the forward or reverse directions depending on whether the actual speed is lower or higher than the theoretical speed respectively.
However, several problems have existed heretofore which have caused erroneous operation of this type of servo motor apparatus. One problem is that the drive pulse width is calculated only as a function of the speed difference signal and does not take into account the actual motor shaft speed. The speed difference signal may have the same value over a wide range of motor shaft speeds. Since the motor torque is lower at high speeds than at low speeds, application of a drive pulse having a predetermined pulse width will have a smaller effect at high speed than at low speeds. Thus, the response of the motor to the speed difference progressively decreases as the shaft speed or velocity increases. This causes insufficiently fast response at high shaft speeds which in turn results in overshooting, undershooting and oscillation.
Another problem is that in such a system the motor shaft is positioned in an open loop manner during the last step and thereafter there is no means of preventing rotation of the shaft from the command position. Thus, the final positioning is inaccurate and movement of a load driven by the motor shaft will result in displacement of the motor shaft from the desired position.